Hammer mills have long been used to reduce the particle size of materials by repeatedly striking particles with a rotating set of hammers and removing small size particles through a screen. FIG. 1 illustrates the structure of a hammer mill 10 and a feeder 41 for pulverizing according to the prior art. In FIG. 1, hammers 26 disposed radially around wheel 30 are rotated counter-clockwise at high speed causing a counter-clockwise air current. An upper portion 20 of a cover (housing) of the hammer mill has a liner 22 on the interior thereof and a lower portion 32 has a mesh screen 34. Particles introduced into a feeder inlet 12 of the feeder 41 are directed to a hammer mill inlet 18 by screw feeder 16. The introduced particles are swept into a space between outer tips of rotating hammers 26 and the liner 22 by the air current and are successively struck by the hammer tips while moving in the hammer mill so that the size of the particles is reduced. The resulting small size particles exit the hammer mill through the mesh screen 34 while particles too large to fall through the screen 34 return to the space between the hammers and the liner to be struck by the rotating hammers.
The hammer mill liner 22 of FIG. 1 includes deflection sections which deflect particles into the path of the hammer tips and thereby increase the force of hammer impact. FIG. 2 illustrates the action of a deflecting portion of the liner 22 on a path 40 of a particle struck by one of rotating hammers 26. As readily seen from FIG. 2, the path of a particle after being struck by the hammer 26 is controlled to follow the path 40 by the deflecting portion of the liner 22. The deflection sections are shaped so that the struck particle is placed back into the path of the rotating hammers. Without the deflection sections, the particles may be entrained by circulating air flow between the hammer tips and the liner of the cover so that the particles are not restruck by the hammers effectively, In order to assure optimum hammer impact, the angle of the liner deflectors and the spacing between the hammer tips and the liner must be accurately controlled. The liner 22 also includes a portion 24 adjacent to the inlet 18 which is spaced further away from the hammer tips to create suction to draw the particles from the inlet 18 into the mill and prevent blow back of the particles. Since the liner is unavoidably to be struck by particles, it is subject to wear and frequent replacement.
As disclosed in U.S. Pat. Nos. 2,291,815 issued to H. E. Korum Aug. 4, 1942 and 3,491,815 issued Jan. 27, 1970 to E. D. Thompson, it is well known to replaceably secure a mesh screen to a portion a hammer mill or similar apparatus by sliding the screen between circular rings or placing the screen on a circular shaped abutment. In prior art hammer mill structures of the type utilizing liners, the upper portion of the housing has been constructed with a varying radius to provide a portion in which particles are induced into the mill chamber and a portion in which the particles are struck by rotating hammers. As a result, it is difficult to employ a sliding type of attaching structure for liners for accurate positioning in a varying radius housing.
Liners of the prior art have been attached to the interior walls of hammer mill covers by regularly spaced bolts 23. FIG. 3 illustrates a deflecting type liner attached to the upper portion cover of a hammer mill by regularly spaced bolts. The bolts, however, must be accurately placed and torqued to control both proper spacing and proper deflection angle. Even minor misalignment of the bolts and a liner causes distortion of the liner and results in incorrect spacing and improper deflection angles. Additionally, the bolts are subjected to repeated particle impact so that they wear rapidly and frequent inspection and replacement are required.